"Supermarkets must do more to end exploitation, pay all their workers a living wage, ensure women get a fair deal and be more transparent about where they source their products."

A separate Oxfam survey of more than 500 workers in the Philippines, Ecuador, Costa Rica, Peru and the US found three quarters of workers saying they were not paid enough to cover basic needs such as food and housing.

More than a third said they were not protected from injury or harm at work and were not able to take a toilet break or have a drink of water when they needed it.

An Oxfam spokesperson said abuses in supermarket supply chains were "endemic".

However, Peter Andrews, head of sustainability at the British Retail Consortium (BRC), said: "Supermarkets in the UK are spearheading actions aimed at improving the lives of the millions of people across the globe who contribute to the retail supply chain.

"Our members are working hard to address existing injustices and continue to collaborate internationally with NGOs [non-governmental organisations], business groups and government on this vital issue."

Meanwhile, Oxfam ranked supermarket giants on their sourcing policies, with all showing an improvement compared with last year.

Tesco, which was at the top of the pile, was given a score of 38%.

A Tesco spokesperson said: "This is the second year in a row that Tesco has been assessed by Oxfam as doing most, of all major supermarkets globally, to ensure human rights are respected in food supply chains."

It said its tea was Rainforest Alliance certified and that it was "committed to improving the lives of tea workers and ensuring minimum working conditions."

It added: "We know there is always more to do and we are working collaboratively with NGOs, trade unions and others to improve wages in the key produce, tea and clothing sectors and ensure working conditions are fair."

An Aldi spokesperson said: "We continue to work hard to ensure every person working in our supply chain is treated fairly and has their human rights respected.

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